Synchronous demodulator

ABSTRACT

A synchronous demodulator ( 10 ) drives an output antenna ( 21 ) with a push-pull driver circuit ( 13 ). The push-pull driver circuit ( 13 ) includes two transistors connected in a push-pull configuration driving a center-tapped transformer ( 17 ). The transformer ( 17 ) couples the push-pull driver circuit ( 13 ) to an antenna ( 21 ). Received signals are demodulated in the output driver circuit ( 13 ).

BACKGROUND OF THE INVENTION

The present invention relates, in general to radio frequency modulationtechniques, and more particularly, to synchronous demodulation of radiofrequency signals.

The recent rise-in popularity and use of contactless credit cards orsmartcards emphasizes the importance of having effective and reliableoperation of such smartcard systems. A typical smartcard system has asmartcard terminal or reader and a portable smartcard that is powered bythe reader through magnetic fields coupled through an antenna on thereader and an antenna on the smartcard. The reader typically transmits aradio frequency signal that is received by the smartcard and is used topower electronics within the smartcard. The smartcard recovers a clockfrom the radio frequency signal and uses the clock to form a subcarrierused to modulate data onto the radio frequency signal. This modulatedradio frequency signal is received by the reader. The reader demodulatesthe modulated radio frequency signal to extract the encoded data. In thepast, various techniques have been used to demodulate the encoded datatransmitted by the smartcard to the reader.

Typically, the reader transmits the RF carrier through an outputantenna. Additionally, the reader has a carrier rejection tuned circuitattached to the antenna of the reader in order to receive the datamodulated RF carrier. This arrangement requires critical and precisetuning because rejection of the power in the carrier inherently requiresa high “Q” filter. This increases the cost of the reader system. Overtime, the frequency adjustment drifts, thus, constant readjustment isrequired to ensure proper operation.

Additionally, the carrier rejection tuned circuit only passes onesideband of the double sidebands in the received RF signal. This resultsin a loss of at least 6 dB, thus, the amplitude of the RF signalrecovered by the tuned circuit is very small, typically less thanapproximately 2.0 milli-volts. Consequently there is a smallsignal-to-noise ratio making it difficult to accurately recover the datafrom the modulated RF carrier.

Further, because the carrier rejection tuned circuit only recovers onesideband, it creates a phase modulation factor that makes it difficultto recover the phase of the received modulated RF signal, thus, makingit difficult to recover data transmitted by the smartcard to the reader

Accordingly, it is desirable to have a demodulation method that does notrequire a critical adjustment to tune to the RF carrier frequency, thatdoes not create phase modulation, and that recovers a large signal fromthe transmitted signal from the card.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a smart card system in accordance withthe present invention;

FIG. 2 schematically illustrates waveforms of the system of FIG. 1 inaccordance with the present invention; and

FIG. 3 schematically illustrates a synchronous demodulation circuit inaccordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a smart card system 100 thatsynchronously demodulates a data signal that is part of a receivedmodulated radio frequency (RF) signal. System 100 includes a smartcardterminal or reader 102 that communicates with a portable smartcard 101.Reader 102 includes an RF transmitter that drives an output antenna 21,and a synchronous demodulator 10 that receives the modulated RF signalfrom antenna 21.

FIG. 2 schematically illustrates waveforms of the system of FIG. 1.Similar elements in FIG. 1 and FIG. 2 have the same element numbers. Awaveform 106 represents the RF carrier transmitted by reader 102 tosmartcard 101. A waveform 107 represents the baseband data signalgenerated by smartcard 101 and that is to be transmitted to reader 102.A waveform 108 represents the subcarrier signal that is developed bysmartcard 101 after smartcard 101 receives the RF carrier signal ofwaveform 106, extracts a the clock from the signal shown by waveform106, and divides the clock down to the subcarrier frequency. Typically,the RF carrier is about 13.56 MHz, and the subcarrier signal is about847.5 kHz. A waveform 109 represents the modulated subcarrier formed bymodulating the subcarrier illustrated in waveform 108 with the basebanddata illustrated in waveform 107. A waveform 110 represents themodulated RF signal received by reader 102.

FIG. 3 schematically illustrates synchronous demodulator 10 thatreceives the modulated RF signal formed by smartcard 101, along withother elements used to transmit the RF carrier to smartcard 101. Similarelements in FIGS. 1, 2, and 3 have the same element numbers. Anoscillator 11 forms an RF frequency, for example 13.56 Mhz, that is usedto develop the transmitted RF carrier. The RF frequency formed byoscillator 11 is applied to an amplifier input 15. An amplifier circuit12 receives the RF frequency and couples it to an output driver circuit13 in order to drive antenna 21 thereby transmitting power in thetransmitted RF carrier to smartcard 101.

Amplifier circuit 12 has a amplifier driver transistor 19 with a baseconnected to input 15 for receiving the RF frequency an emitter coupledto a power return 34 through a resistor 33, and a collector coupled toone side of an amplifier coupling transformer 18 in order to switchtransformer 18 at the RF frequency. Another terminal of transformer 18is connected to a power supply terminal 31. Transformer 18 isolatesamplifier circuit 12 and couples the RF frequency to output drivercircuit 13.

Circuit 13 includes a push pull circuit and an output impedance matchingtransformer 17. The push pull circuit has a first push pull drivetransistor 14 and a second push pull drive transistor 16 that areconnected to opposite terminals of transformer 17. Transformer 17matches the impedance of antenna 21 to transistors 14 and 16. Transistor14 has a base connected to one terminal of transformer 18 in order toreceive the RF frequency, an emitter coupled to return 34 through aresistor 36, and a collector connected to transformer 17 in order todrive one side of transformer 17 at the RF frequency. Transistor 16 hasa base connected to another output of transformer 18, an emitterconnected to return 34 through a resistor 37, and a collector connectedto a second input of transformer 17 in order to drive transformer 17 atthe RF frequency. A center terminal 23 functions as a power supply inputof transformer 17 and is coupled to power supply terminal 31 through aseries resistor 38 and an inductor 24 of a coupling transformer 26. Asecondary winding of transformer 17 is connected to antenna 21 in orderto drive antenna 21 with sufficient energy to power the electronicsconnected to an antenna 105 of card 101.

As the RF frequency from oscillator 11 is applied to transistors 14 and16, a current I_(o) flows through terminal 23 and inductor 24. Whensmartcard 101 is not modulating the RF carrier, current I_(o) has awaveform and frequency that matches the waveform and frequency of the RFcarrier illustrated by waveform 106 in FIG. 2.

After smartcard 101 receives the RF signal from antenna 21, smartcard101 modulates the RF carrier with the data encoded subcarrier to formthe modulated RF signal illustrated by waveform 106. This modulated RFsignal is received by antenna 21 and is coupled back into the secondarywinding, and thus, into the primary windings of transformer 17. Becausetransistors 14 and 16 are switching at the same frequency as themodulated RF signal that is coupled into the primary windings oftransformer 17, the transmitted RF carrier is removed from the modulatedRF signal thereby leaving the data modulated subcarrier signal suppliedby smartcard 101. The both sidebands of the modulated RF are preservedand the energy contained therein is preserved thereby providing a largeramplitude signal, a higher signal-to-noise ratio, and preventing theintroduction of phase modulation into the recovered data modulatedsubcarrier signal. This data modulated subcarrier signal modulatescurrent I_(o) at the same frequency and phase as the data modulatedsubcarrier formed by smartcard 101. Consequently, the primary centretapwinding of transformer 17 and the switching of transistors 14 and 16demodulate the RF carrier from the received modulated RF signal leavingonly the data modulated subcarrier, thus, the 13.56 Mhz RF carrier hasbeen removed leaving the 847.5 kHz data modulated subcarrier signal. Asa result, current I_(o) is also modulated by the data modulatedsubcarrier signal.

As current I_(o) flows through inductor 24, the value and frequency ofcurrent I_(o) is coupled into the secondary winding or inductor 25 oftransformer 26. Inductor 25 and a capacitor 39 form a bandpass filter 27that is tuned to the data modulated subcarrier signal, for example 847.5kHz. Filter 27 typically has a “Q” of less than approximately 40 therebyallowing sufficient bandwidth for the one hundred eighty degree phaseshifting that occurs in the data modulated subcarrier signal.Consequently, synchronous demodulator output 32 has an output signalthat represents the data modulated subcarrier signal formed by smartcard101. Typically, synchronous demodulator output 32 has a signal amplitudethat is at least 500 milli-volts which assists in ensuring accuraterecovery of the data.

Output 32 is coupled to an input of an amplifier 28 that is used toprovide additional gain and shaping to provide a digital signal that isutilized by a digital logic block 29. Digital logic block 29 typicallyrecovers the data from the modulated subcarrier signal. Such digitalrecover circuits are well known to those skilled in the art. Digitallogic block 29 may also performs other digital logic functions on thesignal received from smartcard 101.

It should be noted that demodulator 10 can also be used to demodulate areceived signal that is created by using the data to turn the subcarrieron and off instead of using the data to phase shift modulate thecarrier. Additionally, output driver circuit 13 can be have class B, C,or D amplifier as long as the output driver circuit has a couplingtransformer, such as transformer 26, coupled in series with the powersupply line to the output driver circuit in order to detect variationsin the load current supplied to the output driver circuit. Thesevariations in the load current are detected in the current coupledacross the coupling transformer.

By now it should be appreciated that there has been provided a novelsynchronous demodulation circuit and method. Demodulating the RF carrierfrom the modulated RF signal in the output transformer and outputtransistors assists in ensuring that no phase ambiguities are introducedin the received signal. Coupling a portion of the output current into abandpass filter removes RF signals from the output current and couplesreceived data signals through the input filter with the same phase andfrequency that was used for transmitted the data signals. Consequently,the receiver data signals do not have phase variations and are easilyrecovered by digital logic. This synchronous demodulation techniquesignificantly reduces the amount of circuitry required to recover thedata signal from the received RF modulated carrier signal.

What is claimed is:
 1. A synchronous demodulator comprising: an outputdriver stage having a power supply input; and a coupling transformerhaving one inductor of the coupling transformer operably coupled inseries between the power supply input and a power supply terminal of thesynchronous demodulator for detecting variation in a load current of theoutput driver stage; wherein the output driver stage includes; apush-pull output driver having a first transistor and a secondtransistor; a center-tapped transformer wherein the first transistordrives one terminal of a center-tapped winding, and the secondtransistor drives a second terminal of the center-tapped winding; afirst inductor coupled in series with the power supply input; and asecond inductor operably coupled to develop a voltage selectively inresponse to current flow through the fast inductor corresponding to asub-carrier of a received signal derived by modulating the signal ofsaid output driver stage with said sub-carrier.
 2. The synchronousdemodulator of claim 1 further including a capacitor in parallel withthe second inductor for forming a bandpass filter.
 3. The synchronousdemodulator of claim 2, further including an antenna coupled to theoutput driver stage.
 4. The synchronous demodulator of claim 1, furtherincluding an antenna coupled to the output driver stage.
 5. Asynchronous demodulator comprising: an output driver stage having apower supply input; a coupling transformer having one inductor of thecoupling transformer operably coupled in series between the power supplyinput and a power supply terminal of the synchronous demodulator fordetecting variation in a load current of the output driver stage, and anantenna coupled to the output driver stage said variation in loadcurrent corresponding to a sub-carrier of a received signal derived bymodulating the signal of said output driver stage with said sub-carrier.6. The synchronous demodulator of claim 5 wherein the output driverstage comprises at least one of a class B, C and D amplifier.